V

RF

A =

RI

-

V

1.8

A =

3.6

0.5

= -

-

V

VOUT

A =

VIN

Vsup+

+

VOUT

RF

VIN

RI

Vsup-

9

LF347, LF347B

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SLOS013C – MARCH 1987 – REVISED MARCH 2016

Product Folder Links: LF347 LF347B

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Copyright © 1987–2016, Texas Instruments Incorporated

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

The LF347 has four independent amplifiers that have very low input bias current which allow using higher

resistance resistors in the feedback network. The upper input common mode range typically goes to the positive

supply rail. The lower common mode range does not include the negative supply rail; it must be at least 4-V

greater. Output resistance is 200

Ω to protect the device from accidental shorts.

9.2 Typical Application

A typical application for an operational amplifier is an inverting amplifier. This amplifier takes a positive voltage on

the input, and makes it a negative voltage of the same magnitude. In the same manner, it also makes negative

voltages positive.

Figure 6. Inverting Amplifier

9.2.1 Design Requirements

The supply voltage must be chosen such that it is larger than the input voltage range and output range. For

instance, this application scales a signal of ±0.5 V to ±1.8 V. Setting the supply at ±12 V is sufficient to

accommodate this application.

9.2.2 Detailed Design Procedure

Determine the gain required by the inverting amplifier:

(1)

(2)

When the desired gain is determined, choose a value for RI or RF. Choosing a value in the k

Ω range is desirable

because the amplifier circuit uses currents in the milliamp range. This ensures the part does not draw too much

current. For this example, choose 10 k

Ω for RI which means 36 kΩ is used for R, as determined by Equation 3.

(3)